From D&D Wiki

The metal of a weapon you are holding is desecrated with unholy might, gleams with an onyx black finish from which wisps of black smoke sometimes fall and radiates a evil aura. For the duration, you can use your spellcasting ability instead of Strength for attack and damage rolls on melee attacks using this weapon, and the weapon's damage die becomes 1d8. The weapon also becomes magical, if it isn't already. The spell ends if you cast it again or if you let go of the weapon.

At Higher Levels. When you cast this spell using a level 1 or higher spell slot, change damage to necrotic. When you cast this spell using a level 3 spell slot, increase damage to 2d8. When you cast this spell using a level 6 spell slot, increase damage to 3d8.